In general, some of the energy that results from fuel combustion when an internal combustion engine (hereinafter, simply referred to as an “engine” in some cases) such as a diesel engine is in operation is converted into work rotating a crankshaft while the rest of the energy becomes loss. This loss includes cooling loss that is lost as heat generated in an engine main body, exhaust loss that is released to the atmosphere by exhaust gas, pump loss that results from air intake and exhaust, and mechanical resistance loss. The cooling loss and the exhaust loss account for large portions of the entire loss. Accordingly, it is effective to decrease the cooling loss and the exhaust loss when the fuel economy of the internal combustion engine is to be improved.
However, the cooling loss and the exhaust loss have a trade-off relationship in general. In other words, the exhaust loss increases when the cooling loss decreases, and the cooling loss increases when the exhaust loss decreases. Accordingly, the fuel economy of the engine is improved when a combustion state where the sum of the cooling loss and the exhaust loss decreases can realized.
The combustion state of the fuel (air-fuel mixture) in the engine changes depending on “many parameters affecting the combustion state” such as a fuel injection timing and a turbocharging pressure. Hereinafter, the parameter affecting the combustion state will be simply referred to as a “combustion parameter” in some cases. It is difficult and takes a significant period of adaptation time to have values (combinations) suitable for various operation states obtained in advance by an experiment, simulation, or the like with regard to a plurality of the combustion parameters. Accordingly, methods for systematically determining combustion parameters have been developed.
For example, a control device according to the related art (hereinafter, also referred to as a “conventional device”) calculates a “crank angle at a point in time when half of the total amount of heat resulting during a single combustion stroke is generated (hereinafter, referred to as the “angle of the combustion center of gravity”)”. In a case where the angle of the combustion center of gravity and a predetermined reference value deviate from each other, the conventional device causes the angle of the combustion center of gravity to correspond to the reference value by correcting the fuel injection timing or adjusting the oxygen concentration in a combustion chamber (cylinder) based on EGR ratio adjustment (for example, refer to PTL 1).